Integrated-circuit package for proximity communication

ABSTRACT

Embodiments of a multi-chip module (MCM) are described. This MCM includes a first semiconductor die and a second semiconductor die, where a given semiconductor die, which can be the first semiconductor die or the second semiconductor die, includes proximity connectors proximate to a surface of the given semiconductor die. Moreover, the given semiconductor die is configured to communicate signals with the other semiconductor die via proximity communication through one or more of the proximity connectors. Furthermore, the MCM includes an alignment plate and a top plate coupled to the alignment plate. This alignment plate includes a first negative feature configured to accommodate the first semiconductor die and a second negative feature configured to accommodate the second semiconductor die, and the top plate includes a positive feature. Note that the positive feature is coupled to the first semiconductor die, and the positive feature facilitates mechanical positioning of the first semiconductor die.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is related to pending U.S. patent application Ser. No.11/243,300, filed on Oct. 3, 2005, the contents of which are hereinincorporated by reference.

GOVERNMENT LICENSE RIGHTS

This invention was made with United States Government support underContract No. NBCH3039002 awarded by the Defense Advanced ResearchProjects Administration. The United States Government has certain rightsin the invention.

BACKGROUND

1. Field of the Invention

The present invention relates to techniques for packaging integratedcircuits. More specifically, the present invention relates to amulti-chip module that facilitates inter-chip proximity communication.

2. Related Art

Advances in semiconductor technology presently make it possible tointegrate large-scale systems, which can include hundreds of millions oftransistors, into a single semiconductor chip (or die). Integrating suchlarge-scale systems onto a single semiconductor chip increases the speedat which such systems can operate, because signals between systemcomponents do not have to cross chip boundaries, and are not subject tolengthy chip-to-chip propagation delays. Moreover, integratinglarge-scale systems onto a single semiconductor chip significantlyreduces production costs, because fewer semiconductor chips are requiredto perform a given computational task.

Unfortunately, these advances in semiconductor technology have not beenmatched by corresponding advances in inter-chip communicationtechnology. Semiconductor chips are typically integrated onto a printedcircuit board that contains multiple layers of signal lines forinter-chip communication. However, signal lines on a semiconductor chipare about 100 times more densely packed than signal lines on a printedcircuit board. Consequently, only a tiny fraction of the signal lines ona semiconductor chip can be routed across the printed circuit board toother chips. This problem has created a bottleneck that continues togrow as semiconductor integration densities continue to increase.

Researchers have begun to investigate alternative techniques forcommunicating between semiconductor chips. One promising techniqueinvolves integrating arrays of capacitive transmitters and receiversonto semiconductor chips to facilitate inter-chip communication. If afirst chip is situated face-to-face with a second chip so thattransmitter pads on the first chip are capacitively coupled withreceiver pads on the second chip, the first chip can directly transmitsignals to the second chip without having to route the signals throughintervening signal lines within a printed circuit board.

Capacitive coupling requires precise alignment between the transmitterpads and the receiver pads (which are more generally referred to asproximity connectors), both in a plane defined by the pads and in adirection perpendicular to the plane. Misalignment between thetransmitter pads and the receiver pads may cause each receiving pad tospan two transmitting pads, thereby destroying a received signal. Intheory, for communication to be possible, chips must be aligned so thatmisalignment is less than half of a pitch between the pads. In practice,the alignment requirements may be more stringent. In addition, reducingmisalignment can improve communication performance between the chips andreduce power consumption.

Unfortunately, it can be very challenging to align chips properly.Existing approaches include mechanical mounting structures thatfacilitate self-alignment and/or self-adjustment of pad positions. FIG.1 illustrates one such approach in which negative features, such as etchpits 112, and micro-spheres 114 are used to align semiconductor dies 110(and thus proximity connectors) in a multi-chip module (MCM). Theseetch-pits can be defined photolithographically using a subtractiveprocess (i.e., a photolithographic process that removes material), whichtakes place before, during, or after circuit fabrication on thesemiconductor dies 110. This enables the etch pits 112 to be accuratelyplaced on the semiconductor dies 110 in relationship to circuits and theproximity connectors. Therefore, the photolithographic alignment betweenthe etch pits 112 and circuits establishes precise alignment betweencircuits on the top and bottom semiconductor dies 110.

Note that the alignment in the X, Y, and Z directions, as well as theangular alignment between semiconductor dies 110, depends only on therelative sizes of the etch-pits 112 and the micro-spheres 114, and onthe orientation and placement of the etch pits 112 on the semiconductordies 110. In particular, the lateral alignment between circuits on thesemiconductor dies 110 is achieved in a ‘snap-fit’ manner, provided themicro-spheres 114 are appropriately sized to fit into the etched pits112. Clearly, micro-spheres 114 that are too large do not fit into theetch pits 112, and micro-spheres 114 that are too small do not properlyalign the top and bottom semiconductor dies 110. However, if themicro-spheres 114 sit in the groove of the etch pits 112 correctly (forexample, their equators lie exactly at or higher than the surface of thesemiconductor die 110-1 and exactly at or lower than the surface ofsemiconductor die 110-2) then circuits on the top and bottomsemiconductor dies 110 are precisely aligned. Similarly, alignment inthe Z direction is a function of the photolithographic feature size ofthe etch pits 112, the etch depth of the etch pits 112, and the diameterof the micro-spheres 114.

While this approach is useful and applicable to packaging and assemblyof MCMs that include two or more semiconductor dies 110, it suffers fromthe limitation that the placement of micro-spheres 114 into theetch-pits 112 is not a parallel, wafer-scale process that can be readilyperformed at a foundry. Instead, the micro-spheres 114 are often placedinto individual semiconductor dies 110 after fabrication. Consequently,this approach may add complexity and cost to the process of assemblingMCMs.

Moreover, proximity communication poses addition challenges in thedesign and assembly of MCMs, including: providing power to the chips;effectively cooling the chips; interfacing the MCMs to externalinput/output (I/O) mechanisms; testing; reliability in the presence ofperturbations, such as thermal stress, vibration, and mechanical shock;and the ability to rework MCMs to repair and/or replace components thatdo not work.

Hence, what is needed is a method and an apparatus that facilitatesproximity communication without the problems listed above.

SUMMARY

One embodiment of the present invention provides a multi-chip module(MCM). This MCM includes a first semiconductor die and a secondsemiconductor die, where a given semiconductor die, which can be thefirst semiconductor die or the second semiconductor die, includesproximity connectors proximate to a surface of the given semiconductordie. Moreover, the given semiconductor die is configured to communicatesignals with the other semiconductor die via proximity communicationthrough one or more of the proximity connectors. Furthermore, the MCMincludes an alignment plate and a top plate coupled to the alignmentplate. This alignment plate includes a first negative feature configuredto accommodate the first semiconductor die and a second negative featureconfigured to accommodate the second semiconductor die, and the topplate includes a positive feature. Note that the positive feature iscoupled to the first semiconductor die, and the positive featurefacilitates mechanical positioning of the first semiconductor die.

In some embodiments, the mechanical positioning defines relativepositions of the proximity connectors proximate to the surface of thefirst semiconductor die and the proximity connectors proximate to thesurface of the second semiconductor die. Note that the relativepositions are within a first pre-determined distance in a plane whichincludes the proximity connectors proximate to the surface of the firstsemiconductor die, and the relative positions are within a secondpre-determined distance in a direction which is substantiallyperpendicular to the plane.

In some embodiments, the MCM includes a component coupled to theproximity connectors proximate to the surface of the first semiconductordie and coupled to the proximity connectors proximate to the surface ofthe second semiconductor die. This component may be coupled to the givensemiconductor die using first coupling elements. For example, the firstcoupling elements may include micro-spheres.

In some embodiments, the surface of the first semiconductor die facesthe surface of the second semiconductor die. However, in someembodiments the surface of the first semiconductor die and the surfaceof the second semiconductor die both face in the same direction.

In some embodiments, the proximity communication includes opticalcommunication. Moreover, in some embodiments the proximity connectorsproximate to the surface of the first semiconductor die are capacitivelycoupled to the proximity connectors proximate to the surface of thesecond semiconductor die.

In some embodiments, the positive feature includes a protrusion, and atleast a portion of the protrusion has a pyramidal shape. Moreover, agiven negative feature, which can include the first negative feature orthe second negative feature, includes a depression, and at least aportion of the depression has a pyramidal shape.

In some embodiments, the MCM includes a base plate coupled to thealignment plate, where the first semiconductor die is coupled to thebase plate using second coupling elements which facilitate themechanical positioning. These second coupling elements may includemicro-spheres. In some embodiments, the micro-spheres are positionedinto depressions in the base plate and depressions in the alignmentplate. Moreover, the second coupling elements may facilitate anorientation of the first semiconductor die.

In some embodiments, the base plate is configured to cool from the firstsemiconductor die.

In some embodiments, the first semiconductor die is coupled to thesecond semiconductor die using third coupling elements which facilitatethe mechanical positioning. These third coupling elements may includemicro-spheres.

In some embodiments, the top plate includes first connectors having afirst size on a first surface of the top plate and second connectorshaving a second size on a second surface of the top plate. Note that thetop plate is configured to couple the first connectors to the givensemiconductor die and to couple the first connectors to secondconnectors. Furthermore, the second size is larger than the first size.

Another embodiments provides a method for assembling the MCM. Duringthis process, a first semiconductor die is positioned into a firstnegative feature in the alignment plate in the MCM, where thepositioning involves coupling the first semiconductor die to the baseplate in the MCM using fourth coupling elements. Then, a secondsemiconductor die is positioned into a second negative feature in thealignment plate, where the positioning involves coupling the secondsemiconductor die to the base plate using fifth coupling elements. Next,the first semiconductor die is coupled to the second semiconductor dieusing third coupling elements. Note that given coupling elements, whichcan include the first coupling elements, the second coupling elements,or the third coupling elements, facilitate aligning of proximityconnectors proximate to a first surface of the first semiconductor diewith proximity connectors proximate to a second surface of the secondsemiconductor die.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a block diagram illustrating an existing multi-chip module(MCM).

FIG. 2 is a block diagram illustrating a device that includes proximityconnectors in accordance with an embodiment of the present invention.

FIG. 3 is a block diagram illustrating an MCM that includessemiconductor dies that communicate using proximity communication inaccordance with an embodiment of the present invention.

FIG. 4A is a block diagram illustrating a semiconductor die inaccordance with an embodiment of the present invention.

FIG. 4B is a block diagram illustrating a semiconductor die inaccordance with an embodiment of the present invention.

FIG. 4C is a block diagram illustrating a semiconductor die inaccordance with an embodiment of the present invention.

FIG. 5 is a block diagram illustrating a semiconductor die in accordancewith an embodiment of the present invention.

FIG. 6 is a block diagram illustrating a base plate in accordance withan embodiment of the present invention.

FIG. 7 is a block diagram illustrating a technique for assembling an MCMin accordance with an embodiment of the present invention.

FIG. 8 is a block diagram illustrating an MCM in accordance with anembodiment of the present invention.

FIG. 9A is a block diagram illustrating a portion of an MCM inaccordance with an embodiment of the present invention.

FIG. 9B is a block diagram illustrating a portion of an MCM inaccordance with an embodiment of the present invention.

FIG. 10A is a block diagram illustrating a portion of an MCM inaccordance with an embodiment of the present invention.

FIG. 10B is a block diagram illustrating a portion of an MCM inaccordance with an embodiment of the present invention.

FIG. 11 is a block diagram illustrating an interposer plate in an MCM inaccordance with an embodiment of the present invention.

FIG. 12 is a block diagram illustrating a portion of an MCM inaccordance with an embodiment of the present invention.

FIG. 13 is a block diagram illustrating an MCM in accordance with anembodiment of the present invention.

FIG. 14 is a flow chart illustrating a process for assembling an MCM inaccordance with an embodiment of the present invention.

FIG. 15 is a block diagram illustrating a computer system in accordancewith an embodiment of the present invention.

Note that like reference numerals refer to corresponding partsthroughout the drawings.

DETAILED DESCRIPTION

The following description is presented to enable any person skilled inthe art to make and use the invention, and is provided in the context ofa particular application and its requirements. Various modifications tothe disclosed embodiments will be readily apparent to those skilled inthe art, and the general principles defined herein may be applied toother embodiments and applications without departing from the spirit andscope of the present invention. Thus, the present invention is notintended to be limited to the embodiments shown, but is to be accordedthe widest scope consistent with the principles and features disclosedherein.

Embodiments of a method, a semiconductor die, an MCM, and systems thatinclude the MCM are described. Note that the MCM, which is sometimesreferred to as a macro-chip, includes an array of chip modules (CMs) orsingle-chip modules (SCMs), and a given SCM includes at least onesemiconductor die. Furthermore, the semiconductor die communicates withother semiconductor dies, SCMs, and/or devices in the MCM usingproximity communication of electrical (capacitively coupled) signalsand/or proximity communication of optical signals (which are,respectively, sometimes referred to as electrical proximitycommunication and optical proximity communication). This proximitycommunication occurs via proximity pads or connectors that are locatedon or are proximate to a surface of the semiconductor die.

Alignment of proximity connectors on neighboring or adjacentsemiconductor dies or components is facilitated by features on one ormore surface of the semiconductor dies. For a given semiconductor die,these features may include positive features (which protrude or extendabove a surrounding region) and/or negative features (which arepositioned below or recessed relative to a surrounding region). Notethat the features may be defined using an additive (i.e., amaterial-deposition) and/or a subtractive (i.e., a material-removal)processes. In some embodiments, features on a first semiconductor diemate with or couple to features on a second semiconductor die.Furthermore, in some embodiments positive and/or negative features (suchas a pyramidal-shaped etch pit or slot) are used in combination withinter-chip coupling elements (such as micro-spheres or balls). Forexample, the micro-spheres may be used to align components and/or tocouple power or optical signals to the semiconductor die.

Fabrication of the MCM may involve the use of fluidic self-assembly ofthe semiconductor dies, SCMs, and/or components. In particular, one ormore types of coupling elements may be positioned into features in aportion of the MCM (such as a base plate and/or a semiconductor die)using chemical-based and/or geometry-based selection. For example, thegeometry-based selection may involve selection based on sizes and/orshapes of at least some of the coupling elements. Furthermore, thechemical-based selection may involve chemical bonding (such as ionic,covalent, permanent dipole, and/or van der Waals) of at least some ofthe coupling elements to at least some of the features. This bonding maybe between compounds that include nucleic acids (such asdeoxyribonucleic acid or DNA). In some embodiments, the fluidic assemblyinvolves: gravity, mechanical agitation, an electrostatic driving force,and/or a magnetostatic driving force. Note that this technique forassembling the MCM can be implemented in a wafer-scale process, therebyfacilitating: simpler assembly, rapid assembly (for example, inparallel), and/or lower cost.

Embodiments of the semiconductor die and/or the MCM may be used in avariety of applications, including: telephony, storage area networks,data centers, networks (such as local area networks), and/or computersystems (such as multi-processor computer systems). For example, thesemiconductor die may be included in a switch in a backplane that iscoupled to multiple processor blades, or in a switch that is coupled todifferent types of components (such as processors, memory, I/O devices,and/or peripheral devices). In some embodiments, the semiconductor dieand/or the MCM include at least some of the functionality of a computersystem.

We now describe embodiments of a semiconductor die and an MCM. FIG. 2presents a block diagram illustrating an embodiment of a device 200 thatincludes proximity connectors 212 (which may be capacitive, optical,inductive, and/or conductive-based connectors). Device 200 may includeat least one semiconductor die 210, where semiconductor die 210 mayinclude integrated circuit electronics corresponding to layers depositedon a semiconductor substrate. Note that semiconductor die 210 may bepackaged in an SCM and/or an MCM, where the MCM may include two or moreSCMs. When packaged, for example in the SCM or the MCM, semiconductordie 210 is sometimes referred to as a “chip.”

In one embodiment, the proximity connectors 212 may be located on orproximate to at least one surface of the semiconductor die 210, the SCMand/or the MCM. In other embodiments, the semiconductor die 210, the SCMand/or the MCM may be coupled to the proximity connectors 212. In anexemplary embodiment, the proximity connectors 212 are substantiallylocated at or near one or more corners (proximity connectors 212-1 and212-2) and/or edges (proximity connectors 212-3) of the semiconductordie 210. In other embodiments, proximity connectors 212 may be situatedat one or more arbitrary locations on, or proximate to, the surface ofthe semiconductor die 210. Moreover, while not shown, in someembodiments solder balls or pads (such as C4 pads or bumps) may also beincluded on the surface of the semiconductor die 210. For example, anarray of solder pads may be positioned near the center of the surface.These solder pads may be used to couple signals to the semiconductor die210, including: power, ground (GND), control signals, and/or I/Osignals.

As illustrated for the proximity connectors 212-1, there is a firstpitch 214-1 between adjacent connectors or pads in a first direction (X)216 of the surface and a second pitch 214-2 between adjacent connectorsor pads in a second direction (Y) 218 of the surface. In someembodiments, the first pitch 214-1 and the second pitch 214-2 areapproximately equal.

FIG. 3 presents a block diagram illustrating an embodiment of an MCM 300that includes semiconductor dies 210 that communicate using capacitivelycoupled proximity communication (which is used as an illustration).Semiconductor dies 210 may include proximity connectors or pads 212 thatare located on or proximate to at least surfaces 308 of thesemiconductor dies 210. For example, the proximity connectors 212 may besituated beneath protective layers such that they are located below thesurfaces 308. Moreover, subsets of the proximity connectors 212 may becoupled to transmit circuits 310 (such as transmit drivers) and receivecircuits 312 (such as receivers). One of the transmit circuits 310, atleast a subset of the proximity connectors 212 on the adjacentsemiconductor dies 210, and one of the receive circuits 312 mayconstitute a communication channel. For example, the communicationchannel may include: transmit circuit 310-1, some of the proximityconnectors 212, and receive circuit 312-1. Note that transmit circuits310 and receive circuits 312 may utilize voltage-mode signaling (i.e.,voltage-mode drivers and receivers). Furthermore, semiconductor dies 210may also include wiring and electronics (not shown) to relay the datasignals to additional electronics on the semiconductor dies 210, suchas: logic, memory (for example, a packet buffer memory), I/O ports,demultiplexers, multiplexers, and/or switching elements.

In order to communicate data signals using proximity communication,transmit and receive proximity connectors 212 on adjacent semiconductordies 210 may have, at worst, only limited misalignment, i.e.,substantially accurate alignment. For densely packed proximityconnectors, i.e., proximity connectors 212 having a small spacing orpitch 214 (FIG. 2) between adjacent pads, the alignment between two ormore proximity connectors 212 on adjacent semiconductor dies 210 may bewithin a few microns in the first direction (X) 216 (FIG. 2) and/or afew microns in the second direction (Y) 218 (FIG. 2), where the firstdirection (X) 216 and the second direction (Y) 218 are in a first planeincluding at least some of the proximity connectors 212. The alignmentmay be within a few microns in a third direction (Z) approximatelyperpendicular to the first plane. Note that MCM 300 illustrates amisalignment 314 in the third direction (Z).

In some embodiments, the proximity connectors 212 may be aligned in allsix degrees of freedom, including: the first direction (X) 216 (FIG. 2);the second direction (Y) 218 (FIG. 2); the third direction (z); an anglein the first plane defined by the first direction (X) 216 (FIG. 2) andthe second direction (Y) 218 (FIG. 2); an angle in a second planedefined by the first direction (X) 216 (FIG. 2) and the third direction(Z); and an angle in a third plane defined by the second direction (Y)218 (FIG. 2) and the third direction (Z). Note that X 216, Y 218, and Zare the normal orthogonal axes of 3-space. Also note that if a surface,such as the surface 308-1, of either of the adjacent semiconductor dies210 is non-planar (for example, due to quadrapole distortion) additionalalignment problems may be introduced.

In some embodiments, allowed misalignment in the first direction (X) 216(FIG. 2), the second direction (Y) 218 (FIG. 2), and/or the thirddirection (z) is less than one half of the pitch 214 (FIG. 2) betweenadjacent pads 212. For example, misalignment in the first direction (X)216 (FIG. 2) and/or the second direction (Y) 218 (FIG. 2) may be lessthan 25 μm, and the misalignment 314 in the third direction (z) may beless than 5 μm. In some embodiments, the misalignment 314 is between 1and 10 μm.

Solutions, such as self-aligning and/or self-adjusting of the relativepositions of the proximity connectors 212 on adjacent semiconductor dies210 (and/or in a component such as a bridge chip coupling two or moresemiconductor dies 210) may reduce and/or eliminate the misalignment 314in the third direction (Z). For example, structures that haveflexibility compliance (or are spring-like) may be used. In otherembodiments, a feedback control loop may be used to reduce and/oreliminate the misalignment 314 in the third direction (Z). Moreover, asdiscussed further below, alignment of the semiconductor dies 210 (andthus, at least some of the proximity connectors 212) may be facilitatedby coupling alignment features 316 located on or proximate to thesurfaces 308.

Reducing or eliminating the misalignment 314, in turn, may lead to atleast partial overlap of one or more proximity connectors 212 on theadjacent semiconductor dies 210 and may therefore increase a magnitudeof the capacitively coupled data signals. In addition, the solutions mayreduce misalignment in the first plane, i.e., the plane including atleast some of the proximity connectors 212, when used in conjunctionwith techniques such as electronic steering (where data signals arerouted to given proximity connectors 212 based on the alignment in thefirst plane). Consequently, these solutions may facilitate proximitycommunication between the semiconductor dies 210, SCMs and/or MCMs. Thesolutions may also reduce and/or eliminate a need for narrow tolerances,precise manufacturing, and/or precise assembly of the semiconductor dies210, the SCM, and/or the MCM.

In the embodiments described above and below, the proximity connectors212 on the adjacent semiconductor dies 210 utilize capacitive couplingfor inter-chip communication. In other embodiments, different connectorsmay be overlapped on adjacent semiconductor dies 210. For example, oneembodiment of the present invention uses optical proximity connectors,in which data signals are communicated optically between terminals onadjacent semiconductor dies 210. Moreover, optical waveguides, fibers,light sources (such as diodes or lasers), and/or transceivers may beintegrated onto semiconductor dies 210 (or an accompanying circuitboard) for intra-chip communication. Other embodiments use magneticproximity connectors, in which data signals are communicatedmagnetically between terminals on closely adjacent semiconductor dies210, or conductive connectors (such as an array of solder balls).

In some embodiments, semiconductor dies 210 are contained in an array ofsemiconductor dies in an MCM. For example, as illustrated in FIG. 3,semiconductor dies 210 in such an array may be positioned face-to-face,such that proximity connectors 212 on the corners (and more generally onside edges) of the semiconductor dies 210 overlap and couple signalsbetween adjacent semiconductor dies using, for example, capacitivelycoupled proximity communication. In another embodiment, thesemiconductor dies 210 are face up (or face down) and signals betweenadjacent semiconductor dies are capacitively coupled via a face-down (orface-up) bridge chip.

While the device 200 (FIG. 2) and the MCM 300 are illustrated as havinga number of components in a given configuration, in other embodimentsthe device 200 (FIG. 2) and/or the MCM 300 may include fewer componentsor additional components, two or more components may be combined into asingle component, and/or a position of one or more components may bechanged. Furthermore, functions of the MCM 300 may be implemented inhardware and/or in software.

We now described embodiments of alignment features, such as alignmentfeatures 316. In general, a wide variety of features, including positivefeatures and negative features, may be used. These features may befabricated on a wide variety of materials, including a semiconductor, ametal, a glass, sapphire, and/or silicon dioxide. In the discussion thatfollows silicon is used as an illustrative example. Furthermore, thefeatures may be fabricated using additive and/or subtractive processes,including sputtering, isotropic etching, and/or anisotropic etching. Insome embodiments, features are defined using photolithographic and/ordirect-write techniques.

FIGS. 4A-4C provide embodiments 400, 430, and 450 that illustratenegative features fabricated on semiconductor dies 410, including:trenches, etch pits or slots 412, pyramids 440, and/or truncatedpyramids 460. As noted previously, negative features may be fabricatedusing a subtractive process, for example, by selective etching into asilicon substrate. Note that the etching may be self-limiting orself-terminating, such as anisotropic lithography along the <111>crystallographic direction to produce pyramids 440 (FIG. 4B). However,in some embodiments etch stops are defined, for example, using CMOStechnology, to produce truncated pyramids 460 (in which the sides arealong the <111> crystallographic direction and the bottom is, forexample, along the <100> crystallographic direction). Alternatively,truncated pyramids 460 may be fabricated by stopping an anisotropic etchprior to completion (such as when a desired etch depth is reached).

While not shown, positive features may include: hemispheres, ridges,top-hat shapes or bumps, pyramids, and/or truncated pyramids or mesas.For example, photoresist or metal bumps may be lithographically definedand annealed to allow surface tension to draw the material into ahemisphere (which may be subsequently hard baked). In some embodiments,these features mate with or couple to corresponding negative featuresfacilitating ‘snap-fit’ assembly, thereby providing and maintainingprecise alignment.

While embodiments 400 (FIG. 4A), 430 (FIG. 4B), and 450 (and theembodiments described below) are illustrated as having a limited numberof negative features having a given configuration, other embodiments mayinclude fewer components or additional components, two or morecomponents may be combined into a single component, and/or a position ofone or more components may be changed. For example, the negative and/orpositive features may be fabricated in one or more directions. Thus, insome embodiments, positive features such as hemispheres have ahexagonal-closed-packed configuration. Furthermore, a wide variety ofmaterials may be used for the positive and/or negative features. And insome embodiments, a given semiconductor die includes both positive andnegative features, thereby breaking the symmetry and ensuring that chipscan only be assembled in one physical arrangement or orientation

In some embodiments, a shape of one or more positive and/or a negativefeatures is used to determine an orientation of a semiconductor die orto limit the possible semiconductor dies that a given semiconductor diecan mate with in an MCM (thereby facilitating self-assembly of an MCM).This is illustrated in FIG. 5, which provides a block diagram of anembodiment 500 of a semiconductor die 510 that includes a key-shapedfeature 512. Moreover, in some embodiments an arrangement of one or morefeatures is used to restrict orientation or mating of semiconductordies. This is illustrated in FIG. 6, which provides a block diagram ofan embodiment 600 of a base plate 610 and features 612. Note thatsemiconductor dies and/or components (such as bridge chips) couple tothe base plate 610 during the assembly of an MCM.

In some embodiments, one or more features on the semiconductor diesinclude a material, such as a soft metal to provide stress relief (forexample, for stress due to relative motion or due to temperaturedifferences) between coupled semiconductor dies. Furthermore, metallayers in or on such features may also allow coupling elements (such asmicro-spheres) in an MCM to couple power to one or more semiconductordies. In these embodiments, the coupling elements are made of metal orhave a metal (conductive) coating (such as gold). These couplingelements may or may not be used to align the semiconductor dies. Forexample, in some embodiments alignment is facilitated using positive andnegative features and micro-spheres are used to couple power and/or GNDto the semiconductor dies.

In some embodiments, spherical lenses or micro-spheres are used to alignsemiconductor dies and/or to couple optical signals betweensemiconductor dies. For example, micro-spheres may image light from awaveguide integrated on a first semiconductor die onto a waveguideintegrated on a second semiconductor die, thereby facilitating opticalcommunication between these semiconductor dies. In another embodiment,spherical resonators doped with optional optical gain materials are usedto precisely align the first semiconductor die to the secondsemiconductor die. These spherical resonators may facilitate azimuthalcoupling between the first waveguide integrated on the firstsemiconductor die and the second waveguide integrated on the secondsemiconductor die. Moreover, the spherical resonators may facilitateoptical filtering and optical gain during optical communication betweenthese semiconductor dies.

Thus, the micro-spheres may include materials such as: sapphire, glass,silicon dioxide, conductive materials (for example, a metal), and/ornon-conductive materials.

In the discussion that follows, coupling elements (such asmicro-spheres) are used in conjunction with negative features (as anexample) to align semiconductor dies in an MCM. As noted previously, itis often difficult to place the coupling elements into the featuresduring a wafer-scale process. In principle, fluidic self-assembly may beused to sort and position objects, such as coupling elements, into thefeatures during a wafer-scale process. For example, assembly may bebased on the geometry (i.e., the size, shape, and/or orientation) of thecoupling elements and/or the features. However, while suchgeometry-based techniques offer high directional selectivity (asillustrated in FIGS. 5 and 6), the site selectivity (i.e., the abilityto ensure that a given type of coupling element is placed into orcoupled to a given type of feature) may be limited. This is a challenge,especially in heterogeneous environments that include coupling elementsand/or features that have a range of: sizes, shapes, and/ororientations.

In contrast, chemical-based coatings (for example, adhesion promoterssuch as surfactants) on the coupling elements and/or in the features canoffer high site selectivity. While arbitrary chemical compounds may beused to implement chemical-based fluidic self-assembly, in thediscussion that follows chemicals containing one or more nucleic acidsor nucleotides (such as DNA) are used as an illustrative example.

Nucleotides are composed of a phosphodiester covalently bound to anucleoside or a derivative of a deoxyribose sugar and either a purine orpyrimidine nucleobase. Nucleobases include purines, such as: adenine(A), guanine (G), and the pyrimidines, i.e., thymine (T) and cytosine(C). These nucleotides can be bound to each other to form a linear chain(or strand) through their phosphodiester bonds that must terminate orbegin at either the 5′ or 3′ carbon of the adjacent nucleotide (i.e.,the 5^(th) or 3^(rd) carbon in the deoxyribose sugar). This arrangementimparts a direction to the chain because of an exposed 3′ or 5′ site atopposite ends. Note that each end is capped with either an —OH or aphosphate group.

A sequence of nucleotides (also called bases) in the strand can bearbitrary and by convention is written as a sequence from the 5′ end tothe 3′ end (for example, 5′-AGGTC-3′). This represents a so-calledsingle-stranded DNA molecule. Furthermore, the geometry of thephosphodiester bond and the shape of the nucleosides create thepotential for single strands of DNA to wrap around one another inanti-parallel directions. Thus, any two strands are geometricallycompatible if oriented in an anti-parallel fashion and can form ahelical structure, or a double-stranded DNA molecule.

DNA-assisted self-assembly is a technique in which artificiallysynthesized single-stranded DNA self-assemble into DNA molecules. TheseDNA molecules have ends that display strong affinity for andpreferentially match to the corresponding ends of certain other DNAmolecules, thereby promoting the matching or mating of the moleculesinto a lattice. Note that the self-assembly of large two-dimensionallattices consisting of thousands of molecules has been demonstrated, andeven three-dimensional lattices are expected. This spontaneousself-ordering of sub-structures into super-structures can be a powerfultool for self-assembly of complex systems.

An important quality of DNA that makes suitable for self-assembly is itsability to hybridize with its complement with very high selectivity.Furthermore, the ability to convert double-stranded DNA into a highlyconductive ohmic contact during a metallization process makes the use ofDNA assembly at micro- and nano-length scales useful for establishingcircuit connections. Note that the hybridization or self-assembly isguided by the thermodynamic properties of DNA that give it the abilityto form unique pairs among complementary strands. Also note that thesetechniques may be used to create self-assembling structures at lengthscales between 10 nm (the molecular scale) and a few centimeters withstrong site selectivity. For example, simple experiments have shown thatconductive gold balls can be hybridized using DNA to select specificlocations on an array.

Unfortunately, there are some problems associated with DNA-assistedself-assembly. In particular, self-assembly of nano-scale components maybe hindered by surface-area effects that limit the yield of the process.In other words, there may be competing nonspecific interactions thatneed to be reduced in order to enhance specific (for example,DNA-binding) assembly events. In addition, the assembly of DNA moleculesaccelerates inversely with temperature. Consequently, DNA-assistedself-assembly is an inherently stochastic process with potentiallyuncertain result and the termination of such a process is notguaranteed.

These problems (and those discussed previously) may be addressed bycombining geometry-based selection and chemical-based selection duringfluidic self-assembly to provide high site selection. In someembodiments, a highly selective, stochastic assembly process (such asDNA-assisted self-assembly) includes a strong homogeneous forcingfunction. This assembly process is rapid and parallel (thus, reducingassembly time and cost), and facilitates selective placement ofalignment microstructures (i.e., coupling elements) into correspondingfeatures (such as etch pits) in the host semiconductor dies and/or othercomponents in an MCM (such as the base plate or bridge chips). Inaddition, the combination of these techniques helps terminate theassembly process with high yield and is well suited for heterogeneousassembly.

FIG. 7 presents a block diagram illustrating an embodiment 700 of atechnique for assembling an MCM in which micro-spheres 716 are placedinto corresponding pyramidal-shaped features 712 in a base plate 710. Inthis embodiment, the pyramidal-shaped features 712 include chemicalcoatings 714 and the micro-spheres 716 include chemical coatings 718.These coatings provide chemical-based selectivity. Furthermore, thegeometry of the micro-spheres 716 and/or the pyramidal-shaped features712 provides geometry-based selectivity, as illustrated by the differentsizes of the micro-spheres 716 and the pyramidal-shaped features 712(thus, micro-sphere 716-1 may be positioned into pyramidal-shapedfeatures 712-1 and micro-sphere 716-2 may be positioned intopyramidal-shaped features 712-2). Using this assembly technique,alignment micro-structures or coupling elements (such as themicro-spheres 716) can be self-assembled into the appropriatepyramidal-shaped features 712 in the base plate 710 (such as a siliconchip) with high accuracy and yield. Note that these coupling elementsmay have differing: purposes, materials, sizes, and/or shapes.

In an exemplary embodiment, coatings 714 and/or 718 include one or morenucleic acids or nucleotides, i.e., DNA-assisted fluidic self-assemblyis used to position the micro-spheres 716 into the pyramidal-shapedfeatures 712. This may be accomplished by coating a set ofmicro-structures (such as at least some of the micro-spheres 716) with afirst type of artificially produced DNA strands (i.e., at least some ofthe coatings 718). Then, a photolithographic mask may be used to place asecond set of DNA strands (i.e., at least some of the coatings 714),which are complementary to the first type of DNA strands and have a highaffinity for the first type of DNA strands, into a corresponding set oftarget features (i.e., at least some of the pyramidal-shaped features712) in the base plate 710 where the set of micro-structures are to beassembled.

In some embodiments, these operations are repeated and multiple types ofpairs of coatings are used. For example, a second set ofmicro-structures are coated with a third type of artificially producedDNA strands and another photolithographic mask may be used to place afourth type of artificially produced DNA strands into a correspondingset of target features. Note that the third and fourth types ofartificially produced DNA strands have a high affinity for each otherand may also have a strong repulsion with the first and second types ofartificially produced DNA strands. These operations may be repeateduntil all of the micro-spheres 716 and all of the pyramidal-shapedfeatures 712 include coatings 714 and 718.

In some embodiments of the assembly process, fluids containing differentmicro-spheres 716 (i.e., having different sizes, shapes, and/or coatings718) may be applied sequentially. For example, a first fluid (such as asolvent) containing larger micro-spheres, micro-spheres having a givenshape (such as a cylinder or a sphere), and/or micro-spheres having afirst type of coating may be applied to the base plate 710. This fluidmay remain in contact with the base plate 710 for sufficient time (forexample, a few minutes) to allow these micro-spheres to couple tocorresponding pyramidal-shaped features 712. Then, the fluid may beremoved and any residual or excess micro-spheres, which are on thesurface of the base plate 710 but which are not in or chemically bondedto appropriate pyramidal-shaped features 712, may be removed. Forexample, the fluid may be removed by evaporation and residualmicro-spheres may be removed using a rise or wash operation. Next, theseoperations may be repeated with one or more additional fluids containingprogressively smaller micro-spheres, micro-spheres having another shape,and/or micro-spheres having different types of coatings. Alternatively,in some embodiments the multiple types of micro-spheres are applied tothe base plate 710 in parallel, i.e., using a single fluid.

Note that various metrics may be used to determine how long a givenfluid needs to be in contact with the base plate 710. For example,contact may be maintained until a percentage or all of thepyramidal-shaped features 712 are filled with micro-spheres 716. In someembodiments, a fill factor is determined by measuring how many of thepyramidal-shaped features 712 appear as light or dark in an image (or,more generally, based on a measured reflectivity).

In some embodiments, at least some of the micro-spheres 716 aredissolved after assembly of the MCM is completed. For example, some ofthe micro-spheres 716 may include polystyrene, which may be dissolvedusing acetone or another organic solvent. Moreover, in some embodimentsextra micro-spheres are recovered or recycled from one or more fluidsusing a filtering operation.

In some embodiments, a driving force is used to accelerate the fluidicassembly. For example, an optional driver 720 may apply a DC or timevarying field between a terminal 722 and the base plate 710. In someembodiments, the driving force includes: mechanical agitation (such asultrasound), an electric field, a magnetic field, and/or anelectromagnetic field. Moreover, in some embodiments gravity is used toseparate bound micro-spheres from excess micro-spheres, which may simplyroll of the surface of a tilted base plate 710. Note that the use of adriving force can reduce a sensitivity of the fluidic self-assemblyprocess to temperature variations and/or surface tension.

In an exemplary embodiment, the driving force is an electric field, inan electrochemical transport process referred to asmicro-electrophoresis. However, this technique is only applicable tonon-conductive, homogeneous coupling elements (such as glassmicro-spheres) and movement is only restricted along specific directions(i.e., along the direction of the applied electric field).

Using one or more of these embodiments, an MCM may be assembled withhigh accuracy and high site selectivity. In addition, by combininggeometry-based selectivity with chemical-based selectivity, a stochasticprocess (such as DNA-based fluidic self-assembly) may be converted intoone with a known, high yield.

We now describe embodiments of an MCM. FIG. 8 presents a block diagramillustrating an MCM 800. This MCM (which is sometimes referred to as aRockSolid package) offers improved modularity and functionality, and ishighly manufacturable. In particular, MCM 800 is a multi-layer assemblythat includes: a base plate/cold plate 810, an alignment plate 812, oneor more chips 814 (such as the semiconductor die 210 in FIG. 2), one ormore bridge chips 816 to couple proximity connectors on adjacent chips814, an interposer plate 818, and a top plate 820.

These layers may be coupled together using negative features oralignment pits (such as pyramidal-shaped features 822 and/or truncatedpyramidal-shaped features 824 on the front and/or back surfaces ofcomponents) in conjunction with coupling elements (such as largemicro-spheres or balls 826 and/or fine micro-spheres or balls 828). Notethat the alignment pits in the chips 814 mate with correspondingalignment pits in the base plate/cold plate 810. Thus, the baseplate/cold plate 810 may position the chips 814 with appropriatehorizontal and vertical alignment. In addition, the base plate/coldplate 810 may remove heat from the chips 814. Similarly, the interposerplate 818 and the top plate 820 may provide moderately precise andcompliant horizontal and vertical alignment, as well as power, ground,control signals, and/or I/O signals to the chips 814 and/or the bridgechips 816.

In an exemplary embodiment, the large micro-spheres 826 have a diameterof 300 μm±1 μm and the fine micro-spheres 828 have a diameter of 100μm±0.5 μm. Moreover, the pyramidal-shaped features 822 and the truncatedpyramidal-shaped features 824 may facilitate alignment of the componentswith a tolerance of 5 μm, while the pyramidal-shaped features 830 mayfacilitate alignment of the components with a tolerance of 1 μm. Thus,precise alignment may occur between the bridge chips 816 and the chips814, with the precision limited by the lithographic accuracy of thealignment pits and the diameter of the fine micro-spheres 828 (andindependent of sawing errors that occur when dicing components in theMCM 800).

Note that the MCM 800 facilitates alignment using a hierarchicalapproach (with structures that achieve coarse alignment, and otherstructures that achieve fine alignment). This hierarchical approach:reduces the cost of the components; allows for easier assembly; andincreases the robustness of the alignment to perturbations, such asvibration, thermal stress, and mechanical shock. Furthermore, becausethe critical dimensions only have a few microns of accuracy,manufacturing of some components may be performed with relaxedparticulate counts and/or using low-cost production techniques, such assilk-screening.

While the MCM 800 (and the embodiments described below) is illustratedas having a number of components in a given configuration, in otherembodiments MCM 800 may include fewer components or additionalcomponents, two or more components may be combined into a singlecomponent, and/or a position of one or more components may be changed.For example, in some embodiments the bridge chips 816 are replaced withadditional chips. Proximity connectors on these additional chips arepositioned face-to-face with the proximity connectors on the chips 814in a symmetric (alternating) configuration of chips. However, in someembodiments some of the chips 814 are face-to-face while other chips 814are face up and are coupled to adjacent chips using face-down bridgechips 816.

FIGS. 9A and 9B presents block diagrams 900 and 930 illustrating a sideview and a top view, respectively, of a portion of the MCM 800 in FIG. 8(including base plate/cold plate 810 and alignment plate 812). In thisMCM, the base plate/cold plate 810 may transport heat, generated in thechips 814 (FIG. 8) and/or the bridge chips 816 (FIG. 8), away from theMCM. For example, the base plate/cold plate 810 may include a layer madefrom silicon wafer (the base plate with the alignment pits) positionedabove a lithographically defined damascene copper layer (the coldplate). This damascene copper layer may include micro-channelscorresponding to each chip location, thereby facilitating cooling of thechips 814 (FIG. 8) using a heat-transfer technique, such as liquidand/or air cooling.

Note that the thermal conductivity between chips 814 (FIG. 8) and thebase plate/cold plate 810 may be increased using a variety ofthermal-interface materials (TIMs), including: alloy solder, diamond(such as diamond films produced using chemical vapor deposition), aphase-change material, a thermal oil, a thermal grease, and/or a thermalepoxy. In some embodiments, a TIM is impregnated with a laterallyoriented nano-particle filler that enhances the thermal conductivitybetween a heat source (the chips 814 in FIG. 8) and a heat sink (thebase plate/cold plate 810). Moreover, in some embodiments one or more ofthese approaches may be implemented during a wafer-level process toachieve a low-unit cost.

Alignment plate 812 may be used to provide coarse or sloppy alignment inthe horizontal (X-Y) plane for the chips 814 (FIG. 8) (using negativefeatures, such as wells 940) and the bridge chips 816 (using negativefeatures, such as open-ended slots 942). Moreover, as noted previouslythe base plate/cold plate 810 may align and define an orientation of anarray of chips 814 (FIG. 8) using a pattern of truncated pyramids 824.Also note that the chips 814 (FIG. 8) aligned on the base plate/coldplate 810 will share global positioning with micron-level alignmentaccuracy if the same lithographic process is used to define thealignment pits on the base plate/cold plate 810 and the chips 814 (FIG.8).

FIGS. 10A and 10B presents block diagrams 1000 and 1030 illustrating aside view and a top view, respectively, of a portion of the MCM 800 inFIG. 8 (including base plate/cold plate 810, alignment plate 812, chips814, and bridge chips 816). Block diagram 1030 illustrates the abilityof the baseplate/cold plate 810 to accommodate one or more chip sizesand/or one or more orientations (using one or more bridge chip sizesand/or one or more well sizes) depending on the arrangement of themicro-spheres in the alignment pits. For example, micro-spheres may beselectively coupled to the alignment pits using: a pick-and-placeprocess, the previously described fluidic self-assembly process(including gravity, chemical-based, and/or geometry-based selection),and/or micro-electrophoresis.

Note that the minimum number of alignment pits (or, more generally,negative features) to position and orient a given chip (and to excludechips having different micro-sphere/alignment-pit configurations) istwo. Thus, in some embodiments the base plate/cold plate 810 includesalignment pits and/or patterns of alignment pits that can accommodate avariety of chips 814 (a so-called ‘universal’ base plate). However, insome embodiments the base plate/cold plate 810 includes alignment pitsand/or a pattern of alignment pits that corresponding to a given type ofchip.

In some embodiments chips 814 are thinned and have alignment pits on thefront and back surfaces. Similarly, the bridge chips 816 may also bethinned for improved flexibility, thereby accommodating thicknessvariations in the chips 814. In an exemplary embodiment, the chips 814have a thickness of 200 μm and the bridge chips 816 have a thickness of50-100 μm.

FIG. 10B illustrates a fully populated MCM in which all of the wells 940include chips 814 and all of the chips 814 are coupled to bridge chips816. Note that assembly of this MCM may occur in a series of operations.For example, micro-spheres may be position into alignment pits in one ormore wells 940 during a given operation. Then, the chips 814 may bepositioned into the appropriate wells 940 during one or more additionaloperations. After the chips 814 are positioned, micro-spheres may beposition into alignment pits on a top surface of one or more chips 814and/or in alignment pits in one or more of the slots 942 (FIG. 9B)during one or more subsequent operations. Next, the bridge chips 816 maybe positioned onto appropriate chips 814 during one or more finaloperations. At the end of the assembly process, the chips 814 and thebridge chips 816 will be positioned with sufficient horizontal andvertical accuracy to enable inter-chip proximity communication.

In some embodiments the bottom surfaces of the chips 814 do not includealignment pits, thus providing the chips 814 more lateral freedom (whichis sometimes referred to as a floating-chip configuration). In thiscase, local alignment is accomplished using the bridge chips 816. Forexample, alignment pits and micro-spheres on the top surfaces of thechips 814 may align these chips relative to the bridge chips 816, andthe bridge chips 816 may be aligned relative to the base plate/coldplate 810 using alignment pits and micro-spheres in the slots 942 (FIG.9B).

Note that during assembly, adjacent chips 814 may be repositioned as agiven bridge chip is inserted to lock the array into global alignment.In theory, the alignment can approach one micron (or less). In practicephotolithographic variations may accumulate thereby limiting thealignment to slightly greater than one micron. However, this approachcan tolerate large variations in the positions of the alignment pitswhile maintaining the desired alignment. In particular, the positions ofthe alignment pits may vary by as much as half of the diameter of themicro-spheres and the global alignment of the chips 814 may stillachieve the theoretical limit. This relaxed assembly tolerance lowersthe cost of the components, and thus, of the MCM.

In some embodiments, the base plate/cold plate 810 includes wells 940having different site depths to facilitate positioning adjacent chips814 face up and face down. This arrangement can eliminate some or all ofthe bridge chips 816.

FIG. 11 presents a block diagram illustrating a bottom view of theinterposer plate 1100 in the MCM 800 (FIG. 8). This interposer plate maybe fabricated using silicon, and may include positive features, such asprotruding truncated pyramidal-shaped features or mesas 1110 (which matewith the wells 940 in FIG. 9B), which transmit force and therebycoarsely align the chips 814 (FIG. 8). In addition, the interposer plate1110 may provide power, GND, control signals, and/or I/O signals(henceforth collectively referred to as signals) to the chips 814 (FIG.8) and/or the bridge chips 816 (FIG. 8). For example, connectors 1112may electrically couple the signals to: solder balls, solder pads (suchas an array of C4 pads near the center of the chips 814), and/or vias onthe chips 814 (FIG. 8). Thus, in some embodiments connectors 1112provide power to one side of the chips 814 (FIG. 8) and the resultingheat that is generated is conducted from the other side of the chips 814(FIG. 8) to the base plate/cold plate 810 (FIG. 8).

Note that in some embodiments a given via is implemented using ametal-lined or metal-filled alignment pit, which goes all the waythrough a component (such as a given chip and/or a given bridge chip),in conjunction with a metal or a metal-coated micro-sphere. In someembodiments, vias are fabricated using laser or mechanical drillingfollowed by metallization.

Furthermore, additional connectors or traces (not shown) in theinterposer plate 1100, which are positioned over the slots 942, mayelectrically couple the signals to: solder balls, solder pads, and/orvias on the bridge chips 816 (FIG. 8). Alternatively, the signals may becoupled to the bridge chips 816 (FIG. 8) from the chips 814 (FIG. 8)using metal or metal-coated micro-spheres.

In some embodiments, the signals may be coupled from the top surface ofthe interposer plate 1100 to the bottom surface (shown in FIG. 11) usingconnectors 1114. These connectors may include vias that include ahemispherical, metal-coated protrusion (such as one made of silicongel), which may be fabricated using injection molding or epoxydispensing techniques. These protrusions may transmit force and providecompliance in the vertical direction in the MCM 800 (FIG. 8). In someembodiments, additional protrusions (not shown) over the bridge chips816 (FIG. 8) (i.e., in the slots 942) also transmit force and providecompliance in the vertical direction. Furthermore, connectors 1112 maybe coupled to connectors 1114, for example, using traces (not shown)along with solder or gold paste.

In some embodiments, connectors 1112 include a flexible, interdigitedarray of springs that are configured to electrically couple to 80-μmwide C4 pads on the chips 814 (FIG. 8). For example, each of theconnectors 1112 may include 176,000 springs that are 6 μm wide and whichhave a pitch of 10 μm. These springs may couple to 8500 C4 pads on agiven chip, thereby providing considerable redundancy (and thus,reliability). Furthermore, the springs may be coated with: gold, solder,and/or solder paste. As discussed further below, after testing (and anyreworking) of the MCM 800 (FIG. 8) is completed, this solder may bereflowed to make connections more reliable and/or permanent. However,note that to maintain the compliance of the MCM 800 (FIG. 8), solder maynot be used over the bridge chips 816 (FIG. 8).

In an exemplary embodiment, the interposer plate 1100 has a thickness of600 μm. Moreover, the power to a given chip may be 50-100 W, and thepower to a given bridge chip may be 1 W.

FIG. 12 presents a block diagram 1200 illustrating a side view of aportion of the MCM 800 in FIG. 8 (including interposer plate 818 and topplate 820). Top plate 820 may electrically couple the signals to pads1212 on the top surface of the interposer plate 818 using springs 1210.Note that these springs expand when compressed, and the edges of thesprings 1210 may remove oxide from the surface of the pads 1212. In anexemplary embodiment, the top plate 820 is made of plastic or ceramic,and the springs 1210 are made of a beryllium-gold alloy.

As noted previously, vias in the interposer plate 818 may couple thesignals from the pads 1212 to the bottom surface of the interposer plate(which is illustrated in FIG. 11). Note that the pads 1212 may be largerthan corresponding pads or connectors on the bottom surface of theinterposer plate 818. Thus, the interposer plate 818 may perform aregistration process in which signals are coupled from a region ofcoarse alignment in the MCM 800 (FIG. 8) to a region of fine alignment.For example, the pads 1212 may have a width of 0.5-1 mm and a spacing of0.25 mm (corresponding to an external ball grid array or BGA locatedabove the top plate 820, which is coupled to the springs 1210), whilepads or connectors on the bottom surface of the interposer plate 818 mayhave a width of 80 μm and a spacing of 40 μm (i.e., an order ofmagnitude smaller).

FIG. 13 presents a block diagram illustrating a side view of anassembled MCM 1300. In this MCM, a compression lid 1310 provides aclamping mechanism that holds the assembly together and, via metallizedvias, additionally couples the signals from the top plate 820 to theexternal BGA so that the MCM 1300 will be compatible with existing BGApacking. For example, a grid of solder balls (not shown) may bepositioned on top of the compression lid 1310. Note that by reflowingthese solder balls the MCM 1300 may coupled to a circuit board.

Compression lid 1310 provides a clamping force that holds the completeassembly, from the base-plate/cold plate 810 to the top plate 820, inintimate contact. Consequently, micro-spheres are held in the alignmentpits, the chips 814 are held in the wells 940 (FIG. 9B), and the chips814 and the bridge chips 816 are all clamped in their respective lockedpositions and precise global alignment is maintained. Furthermore,lateral and vertical displacement of the internal components is reducedunder typical vibration and mechanical shock conditions. Thiscompression lid also maintains low resistance electrical connectionsbetween pads, springs, and connectors in the MCM 1300. And thecompression lid 1310 provides environmental protection for the internalcomponents, thereby guarding against exposure to: humidity, corrosivevapors, and debris.

In an exemplary embodiment, the MCM 1300 has a thickness of 2.5 mm.Moreover, the compression lid 1310 provides a uniform force of 1-2 kg or1-5 pounds. For example, the compression lid 1310 may have catenary orparabolic shape prior to assembly. This bowed equilibrium shape helps todistribute the force uniformly over the entire array of internalcomponents in the MCM 1300. In some embodiments, bolts (not shown) arealso used to secure the MCM 1300.

Note that in some embodiments the compression lid 1310 includes one ormore holes to allow an optical fiber and/or a ribbon cable to access theMCM 1300. Moreover, holes may also be etched in the top plate 820 and/orthe interposer plate 818. In some embodiments, the cable may be coupledto: voltage regulators, memory (such as random access memory), and/orstorage devices (such as disc drives).

In the discussion thus far, the components in the MCM 1300 do not needto be soldered or permanently connected to each other. This flexibilityallows the MCM 1300 to be reworked during testing (for example,defective components may be replaced by partially disassembling the MCM1300). Note that, because circuits on one or more chips 814 in the MCM1300 may implement the functionality of an entire computer system,self-testing of the MCM 1300 may be more thorough, thereby offeringimproved identification of (any) defective components.

We now describe embodiments of a process for assembling an MCM. FIG. 14provides a flow chart illustrating a process 1400 for assembling an MCM.During this process, a first semiconductor die is positioned into afirst negative feature in an alignment plate in the MCM (1410), wherethe positioning involves coupling the first semiconductor die to thebase plate in the MCM using first coupling elements. Then, a secondsemiconductor die is positioned into a second negative feature in thealignment plate (1412), where the positioning involves coupling thesecond semiconductor die to the base plate using second couplingelements. Next, the first semiconductor die is coupled to the secondsemiconductor die using third coupling elements (1414). Note that givencoupling elements, which can include the first coupling elements, thesecond coupling elements, or the third coupling elements, facilitatealigning of proximity connectors proximate to a first surface of thefirst semiconductor die with proximity connectors proximate to a secondsurface of the second semiconductor die.

In some embodiments of the processes 1400 there may be additional orfewer operations, the order of the operations may be changed, and two ormore operations may be combined into a single operation.

Note that the MCMs described previously may be included in systems. Forexample, FIG. 15 presents a block diagram illustrating an embodiment ofa computer system 1500, which includes one or more processors 1510, acommunication interface 1512, a user interface 1514, and one or moresignal lines 1522 coupling these components together. Note that the oneor more processing units 1510 may support parallel processing and/ormulti-threaded operation, the communication interface 1512 may have apersistent communication connection, and the one or more signal lines1522 may constitute a communication bus. Moreover, the user interface1514 may include: a display 1516, a keyboard 1518, and/or a pointer,such as a mouse 1520.

Computer system 1500 may include memory 1524, which may include highspeed random access memory and/or non-volatile memory. Morespecifically, memory 1524 may include: ROM, RAM, EPROM, EEPROM, FLASH,one or more smart cards, one or more magnetic disc storage devices,and/or one or more optical storage devices. Memory 1524 may store anoperating system 1526, such as SOLARIS, LINUX, UNIX, OS X, or WINDOWS,that includes procedures (or a set of instructions) for handling variousbasic system services for performing hardware dependent tasks. Memory1524 may also store procedures (or a set of instructions) in acommunication module 1528. The communication procedures may be used forcommunicating with one or more computers and/or servers, includingcomputers and/or servers that are remotely located with respect to thecomputer system 1500.

Memory 1524 may also include the one or more program modules (of sets ofinstructions) 1530. Instructions in the program modules 1530 in thememory 1524 may be implemented in a high-level procedural language, anobject-oriented programming language, and/or in an assembly or machinelanguage. The programming language may be compiled or interpreted, i.e.,configurable or configured to be executed by the one or more processingunits 1510.

Computer system 1500 may include one or more macro-chips 1508 (such asone or more MCMs) that include semiconductor dies and/or components asdescribed in the previous embodiments.

Computer system 1500 may include fewer components or additionalcomponents, two or more components may be combined into a singlecomponent, and/or a position of one or more components may be changed.In some embodiments, the functionality of the computer system 1500 maybe implemented more in hardware and less in software, or less inhardware and more in software, as is known in the art.

Although the computer system 1500 is illustrated as having a number ofdiscrete items, FIG. 15 is intended to be a functional description ofthe various features that may be present in the computer system 1500rather than as a structural schematic of the embodiments describedherein. In practice, and as recognized by those of ordinary skill in theart, the functions of the computer system 1500 may be distributed over alarge number of servers or computers, with various groups of the serversor computers performing particular subsets of the functions. In someembodiments, some or all of the functionality of the computer system1500 may be implemented in one or more application specific integratedcircuits (ASICs) and/or one or more digital signal processors (DSPs).

The foregoing descriptions of embodiments of the present invention havebeen presented for purposes of illustration and description only. Theyare not intended to be exhaustive or to limit the present invention tothe forms disclosed. Accordingly, many modifications and variations willbe apparent to practitioners skilled in the art. Additionally, the abovedisclosure is not intended to limit the present invention. The scope ofthe present invention is defined by the appended claims.

1. A multi-chip module (MCM), comprising: a first semiconductor die anda second semiconductor die, wherein a given semiconductor die, which canbe the first semiconductor die or the second semiconductor die, includesproximity connectors proximate to a surface of the given semiconductordie, and wherein the given semiconductor die is configured tocommunicate signals with the other semiconductor die via proximitycommunication through one or more of the proximity connectors; analignment plate including a first negative feature configured toaccommodate the first semiconductor die and a second negative featureconfigured to accommodate the second semiconductor die; and an top platecoupled to the alignment plate, wherein the top plate includes apositive feature; wherein the positive feature is coupled to the firstsemiconductor die; and wherein the positive feature facilitatesmechanical positioning of the first semiconductor die.
 2. The MCM ofclaim 1, wherein the mechanical positioning defines relative positionsof the proximity connectors proximate to the surface of the firstsemiconductor die and the proximity connectors proximate to the surfaceof the second semiconductor die, wherein the relative positions arewithin a first pre-determined distance in a plane which includes theproximity connectors proximate to the surface of the first semiconductordie, and wherein the relative positions are within a secondpre-determined distance in a direction which is substantiallyperpendicular to the plane.
 3. The MCM of claim 1, further comprising acomponent coupled to the proximity connectors proximate to the surfaceof the first semiconductor die and coupled to the proximity connectorsproximate to the surface of the second semiconductor die.
 4. The MCM ofclaim 3, wherein the component is coupled to the given semiconductor dieusing coupling elements.
 5. The MCM of claim 4, wherein the couplingelements include micro-spheres.
 6. The MCM of claim 1, wherein thesurface of the first semiconductor die faces the surface of the secondsemiconductor die.
 7. The MCM of claim 1, wherein the surface of thefirst semiconductor die and the surface of the second semiconductor dieboth face in the same direction.
 8. The MCM of claim 1, wherein theproximity communication includes optical communication.
 9. The MCM ofclaim 1, wherein the proximity connectors proximate to the surface ofthe first semiconductor die are capacitively coupled to the proximityconnectors proximate to the surface of the second semiconductor die. 10.The MCM of claim 1, wherein the positive feature includes a protrusion,and wherein at least a portion of the protrusion has a pyramidal shape.11. The MCM of claim 1, wherein a given negative feature, which caninclude the first negative feature or the second negative feature,includes a depression, and wherein at least a portion of the depressionhas a pyramidal shape.
 12. The MCM of claim 1, further comprising a baseplate coupled to the alignment plate, wherein the first semiconductordie is coupled to the base plate using coupling elements whichfacilitate the mechanical positioning.
 13. The MCM of claim 12, whereinthe coupling elements include micro-spheres.
 14. The MCM of claim 13,wherein the micro-spheres are positioned into depressions in the baseplate and depressions in the alignment plate.
 15. The MCM of claim 12,wherein the coupling elements further facilitate an orientation of thefirst semiconductor die.
 16. The MCM of claim 1, wherein the base plateis configured to cool from the first semiconductor die.
 17. The MCM ofclaim 1, wherein the first semiconductor die is coupled to the secondsemiconductor die using coupling elements which facilitate themechanical positioning.
 18. The MCM of claim 17, wherein the couplingelements include micro-spheres.
 19. The MCM of claim 1, wherein the topplate includes first connectors having a first size on a first surfaceof the top plate and second connectors having a second size on a secondsurface of the top plate; wherein the top plate is configured to couplethe first connectors to the given semiconductor die and to couple thefirst connectors to second connectors; and wherein the second size islarger than the first size.
 20. A method for assembling a multi-chipmodule (MCM), comprising: positioning a first semiconductor die into afirst negative feature in an alignment plate in the MCM, wherein thepositioning involves coupling the first semiconductor die to a baseplate in the MCM using first coupling elements; positioning a secondsemiconductor die into a second negative feature in the alignment plate,wherein the positioning involves coupling the second semiconductor dieto the base plate using second coupling elements; and coupling the firstsemiconductor die to the second semiconductor die using third couplingelements, wherein given coupling elements, which can include the firstcoupling elements, the second coupling elements, or the third couplingelements, facilitate aligning of proximity connectors proximate to afirst surface of the first semiconductor die with proximity connectorsproximate to a second surface of the second semiconductor die.